1. Field of the Invention
This invention relates to an optical scanning system for correcting the tilt of a reflecting surface, as employed in a laser beam printer or a laser engraving process and, more particularly, to an optical scanning system with a tilt correction function for correcting nonuniformity in the pitch between the scanning lines, due to the inclination of a deflecting surface of a polygon mirror, which acts as a beam reflector in the optical scanning system.
2. Description of the Prior Art
Heretofore, a scanning optical system for effecting a beam scanning over an image plane has widely been known, wherein a light beam from a light source such as a laser beam collimated by a first focusing optical system is caused to mechanically scan or an image plane through a second focusing optical system which focuses the scanning beam onto the image plane.
The deflecting surfaces or facets of the rotating polygon mirror may sometimes be tilted relative to the axis of rotation, or the axis of mounting, of the polygon mirror, in spite of the fact that the deflecting surface and the mirror are originally intended to be aligned parallel to each other. Therefore, such a tilt in the deflecting surface during the scanning, which is commonly called a tilt surface problem, results in nonuniform pitches between the lines on the image plane in the direction (hereinafter referred to as a sub-scan direction) perpendicular to the direction of the main scan, thereby adversely affecting the quality of the image formed by the scanning.
In order to optically correct the tilt of the deflecting surface and enhance the quality of a final image, there have been proposed various optical systems, in which a deflecting surface and an image plane are positioned in a conjugate plane that is perpendicular to the scan plane.
Examples of such an optical system include that shown in Japanese Unexamined Patent Publication No. 48(1973)-98844 which utilizes an optical system positioned between a deflecting surface and an image plane, composed of, from the deflecting surface, a spherical lens which has the distortion characteristics for achieving a uniform scan and a lengthy cylindrical lens.
Japanese Unexamined Patent Publication No. 61(1986)-84620 is illustrative of an optical system including a cylindrical mirror instead of the lengthy cylindrical lens.
Moreover, Japanese Unexamined Patent Publication Nos. 56(1981)-36622, 57(1982)-35823, 63(1988)-50814 disclose an optical system using a lens, which has a toric surface for correcting distortions due to the tilt of the reflecting surface, as an element in the system that has distortion characteristics for effecting a uniform scan.
However, the arrangement of the lenses disclosed in the foregoing Publication No. 48(1973)-98844, i.e., consisting of the spheric lens system and the lengthy cylindrical lens, requires the cylindrical lens to be positioned in proximity to the image plane in order to correct a field curvature. Hence, the cylindrical lens becomes longer as the increase of the scan range, thereby deterring a reduction, in size, of the optical system.
Meanwhile, the optical system disclosed in the Publication No. 61(1986)-84620 seems to have attained nominal improvements in view of the amount of field curvature as compared with the optical system using the cylindrical lens.
This optical system also permits the cylindrical mirror to be positioned at a greater distance from the image plane when compared with the optical system employing the cylindrical lens, whereby it is believed that the optical system may be reduced in dimensions. However, this optical system is restricted in arrangement because of the deflection of light beam by means of the cylindrical mirror.
The optical lens systems shown in the Publication Nos. 56(1981)-36622, 57(1982)-35823, 63(1988)-50814 are reduced in size by use of a lens, as an element in the optical system, having a toric surface which corrects distortions due to the tilt of the surface, thereby obviating the need for the lengthy cylindrical lens or the lengthy cylindrical mirror set forth in the above.
However, according to this optical system, the arrangement of the system that comprises two single lenses renders the back focus of the lens system longer in the scan plane than its focal length. The two lenses are, from the deflecting surface, a first single lens having either a spherical or cylindrical surface and a second single lens having a toric surface and a positive refracting power in the scan plane. Therefore, as a whole, the optical system cannot expect much reduction of its size by shortening the optical path between the optical scanning system and the image plane.